It is often necessary to switch "on" and "off" a circuit for driving an inductive load. A "pre-driver" circuit, is often used for this switching function.
When the inductive load is switched "off", it generates a "flyback" voltage waveform having a large voltage peak. The peak of the flyback voltage waveform can be in the tens or hundreds of volts. Thus, the elements of the pre-driver, such as the switching transistors, must have a breakdown voltage high enough to withstand the large voltage peaks associated with the flyback voltage waveform. If their breakdown voltages are not high enough, the large voltage peaks may permanently damage the transistors.
A problem arises when it is desired to implement a pre-driver that will be exposed to large flyback voltage peaks. Very often, standard or off-the-shelf transistors have too low a breakdown voltage to withstand the flyback voltages generated by the inductive load drivers that they are designed to switch.
One solution to this problem is to use transistors having a breakdown voltage large enough to withstand the generated flyback voltage peaks. However, such transistors may be substantially more expensive than standard transistors. Additionally, such transistors may be in short supply in the market place, making them more difficult to obtain than standard transistors. Finally, increasing the breakdown voltage of a transistor may also cause undesirable changes in other characteristics of the transistor. For example, the physical size of the transistor may be increased, or its operating speed may be reduced.
The problem is compounded when it is desired to implement the pre-driver as an integrated circuit. Again, the standard transistors used within the integrated circuit may not have high enough breakdown voltages to withstand the expected flyback voltage peaks, and increasing transistor size is not an option.
A need has thus arisen for a pre-driver circuit that will not be damaged by a flyback voltage and that can be implemented without altering the breakdown voltages of its transistor components.